Project I aims at the elucidation of the tertiary structure of fibrinogen. We have been able to produce crystalline preparations of both N-DSK and fibrinogen. In both cases the crystals are too small for X-ray crystallography. By modification of the conditions for crystallization we hope to obtain such crystals. Project II deals with isolation and characterization of degradation products of fibrinogen, which influence cell function. Degradation products obtained with plasmin inhibit mitogen induced thymidin incorporation into lymphocytes in vitro. A fragment obtained by cleavage of fibrinogen with CNBr was found to have a pronounced effect in the system. In vivo experiments of this fragment failed to show any certain immunosuppressive action. Mode of action of fibrinogen fragments on different cell types will be investigated. Project III deals with isolation and characterization of the factor VIII complex. A new procedure has been worked out for production of high potency concentrates containing factor VIII:C activity in a yield of 30-50% and F VIII:RCF activity in a yield of about 60%. These concentrates will be tested in clinical trials. Experiments have been designed for isolation and characterization of F VIII:C and F VIII:RCF.